1. Technical Field of the Invention
The present invention relates generally to electric generators and electric power generation systems.
More particularly, the invention relates to a control apparatus for an automotive alternator, which has a capability to accurately detect the temperature of each component of the automotive alternator.
2. Description of the Related Art
An existing control apparatus for an electric generator is disclosed, for example, in Japanese Patent First Publication No. H04-229100, an English equivalent of which is U.S. Pat. No. 5,198,744.
The existing control apparatus detects the temperature of the control apparatus itself, the rotational speed (or, the number of revolutions) of the electric generator, and the duty of an intermittent exciting current supply to the electric generator. Based on all of the detected parameters, the control apparatus further determines the temperature of each component of the electric generator. When the determined temperature is above a predetermined upper limit, the control apparatus limits the power output of the electric generator through limiting the exciting current supply, thereby preventing overheating of the electric generator.
However, with the existing control apparatus, it may be difficult to accurately determine the temperature of each component of the electric generator.
More specifically, to accurately determine a temperature of the electric generator, it is necessary to first accurately determine the ambient (or outside) air temperature and the increasing amount of the temperature of the electric generator due to the power generating operation.
Since the temperature of the control apparatus depends on the ambient air temperature, the exciting current, and the rotational speed of the electric generator, it is possible to reversely determine the ambient air temperature based on the temperature of the control apparatus, the exciting current, and the rotational speed of the electric generator.
However, in the existing control apparatus, the duty of the intermittent exciting current supply to the electric generator is used, instead of the exciting current. Since the resistance of the exciting winding changes with temperature, it is impossible to accurately determine the exciting current based on the duty of the intermittent exciting current supply. Consequently, it is impossible to accurately determine the ambient air temperature, resulting in a large deviation between the determined and actual temperatures of the electric generator.
Moreover, during transient states, the temperature of the electric generator generally changes more gradually than that of the control apparatus.
However, such a fact is not considered in the existing control apparatus. Consequently, when the temperature of the control apparatus quickly rises due to a high load imposed on the electric generator for a short time, the power output of the electric generator may be limited despite the actual temperature of the electric generator having not yet reached to the predetermined upper limit.
Furthermore, without high accuracy in determination of the temperature of the electric generator, it is impossible for the control apparatus to accurately determine other parameters, such as the driving torque of the electric generator and the output current of the electric generator, based on the determined temperature of a stator winding of the electric generator. As an alternative, special detecting devices or sensors may be included in the control apparatus to accurately detect those parameters; however, this will increase the manufacturing cost of the control apparatus.